The present invention generally relates to recording of information signals on magneto-optical recording media, and in particular to an optical recording system for recording an information signal selectively on both sides of a disk-shaped magneto-optical recording medium.
In magneto-optical recording of an information signal, the information signal is recorded on an magneto-optical disk which is a recording disk deposited with a magnetic material by means of an optical beam. The optical beam heats the magnetic material under an external magnetic field and the magnetic material is magnetized in the direction of the magnetic field upon cooling. The reading of the recorded information is made by irradiating the optical beam on the surface of the disk. Upon reflection, the plane of polarization of the optical beam is rotated in a first direction or second direction responsive to the direction of magnetization of the magnetic material as a result of the electrooptic Kerr effect, and the recorded information is reproduced by detecting the change in the plane of polarization of the reflected optical beam.
Generally, conventional magneto-optical disk recording/reproducing apparatus records and reproduces an information signal on one side of a disk-shaped magneto-optical recording medium (referred to hereinafter as a disk) by means of an optical recording/reproducing system. Such an optical recording/reproducing system comprises a light source, an optical system including lenses and mirrors, focusing and tracking mechanisms, and light detection elements. The optical recording/reproducing system is sometimes referred to as an optical head and focuses an optical beam on the disk and receives the optical beam reflected back from the disk. In such a disk, it is naturally desirable that the both sides of the disk can be used for storage of the information signal so that the recording capacity per one disk is increased. However, because of the expensive optical elements used in the optical recording/reproducing system, the prior art magneto-optical recording and reproducing system generally use one such optical recording/reproducing system and the recording is made only on one side of the disk.
In order to enable the recording and reproducing of the information signal on and from the both sides of the disk, one may use a well known automatic turn-over mechanism which turns the disk upside down while using a single optical head for focusing the optical beam on the lower side of the disk, for example. However, such automatic turn-over mechanism is complex and occupies a large space not only with respect to mechanism itself but also due to the movement of the disk at the time of turn-over. Therefore, the magneto-optical disk recording and reproducing apparatus using such automatic turn-over mechanism inevitably becomes bulky and complex.
Also, there is a known construction for an optical disk recording apparatus as proposed in the Laid-open Japanese patent application No. 54-35707, in which a pair of optical systems are provided on both sides of a disk for recording the information signal on the both sides of the disk. In applying the construction of this known optical disk recording apparatus to the recording and reproducing of the information signal on the both sides of a magneto-optical disk, two expensive as well as bulky and heavy optical recording/reproducing systems have to be used. Therefore, the apparatus inevitably becomes expensive, large and heavy. Further, such an apparatus has a problem of a large access time which is a time required for the optical recording/reproducing system to move to a desired track position, as a result of the relatively large inertia of the optical recording/reproducing system. Furthermore, the magneto-optical recording and reproducing apparatus requires a permanent magnet or solenoid on the opposite side of the disk for producing the magnetic field which is used for magnetization of the magnetic material at the time of recording. Thus, the construction as proposed in the Laid-open Japanese patent application can not be used for a recording/reproducing apparatus for recording and/or reproducing the information signal on both sides of the disk as the other side of the disk is occupied by the optical recording/reproducing system and no space is available for such a magnet or solenoid.